1. Field of the Invention
The present invention relates to an optical writing device, an image forming apparatus, a method of controlling the optical writing device, and a computer program product, and more particularly, to correction of an error in installation of an optical writing device that exposes a photosensitive element.
2. Description of the Related Art
Recently, digitization of information tends to be promoted, and image forming apparatuses, like printers and facsimile machines used for output of digitized information and scanners used for digitization of documents, have become essential apparatuses. Many image forming apparatuses include imaging functions, image forming functions, communication functions, and the like, to be configured as multifunction peripherals that are usable as printers, facsimile machines, scanners, and copiers.
Of these image forming apparatuses, electrophotographic image forming apparatuses are widely used as image forming apparatuses used for output of digitized documents. An electrophotographic image forming apparatus forms an electrostatic latent image on a photosensitive element by exposing a photosensitive element, and develops the electrostatic latent image into a toner image using a developer such as toner, and transfers the toner image onto a sheet of paper to output the sheet of paper.
In such an electrophotographic image forming apparatus, an image to be output is divided into lines, and image formation is performed per line. In general, an optical writing device, which emits a laser beam, exposes a photosensitive element to a laser beam per line to form an electrostatic latent image. As a light source of such an optical writing device, a light-emitting diode print head (LPH) may be used.
Problems that optical writing devices such as LPHs have include a problem of an error in their installation in image forming apparatuses. An LPH is mechanically installed to an image forming apparatus, and in this mechanical installation, the LPH may be installed with a tilt from its proper state, or an installation position may be tilted due to causes such as vibrations caused by an operation of the apparatus, a change in temperature, a change over time, or the like. These are generally called “skew”. If the LPH is tilted from the proper state, an output light from a light-emitting element is not accurately emitted to an image forming position on a photosensitive element exposed by the LPH. This may cause a reduction in image quality, a color shift among C (cyan), M (magenta), Y (yellow), and K (blacK) colors, or the like.
Against this problem, a technique for correcting a tilt by setting main scanning dividing positions in advance and employing a candidate dividing position closest to a set dividing position as a dividing position in a main scanning direction has been proposed (for example, see Japanese Patent Application Laid-open No. 2009-27683). Furthermore, a method for correcting a skew by allocating image data to banks to write them in a memory table has been proposed (for example, see Japanese Patent Application Laid-open No. 2008-36850).
In a typical LPH, semiconductor chips each including light-emitting elements integrated together are mounted on a substrate to satisfy a width in a main scanning direction. For example, 192 dots of light-emitting elements are integrated on each semiconductor chip at intervals of 42.3 micrometers, and 26 of this semiconductor chip are mounted on a substrate, whereby an LPH of 600-dpi in A4 size is formed.
In this LPH, as described above, one semiconductor chip is generated by integrating light-emitting elements, so a positional deviation does not become a problem among the light-emitting elements in that one semiconductor chip. However, when plural semiconductor chips are mounted on a substrate, a positional deviation among the semiconductor chips may occur. Therefore, in addition to a tilt upon installation of an LPH in an image forming apparatus, the positional deviation among the semiconductor chips inside the LPH may cause the above described reduction in image quality, color shift, and the like.
In each of Japanese Patent Application Laid-open No. 2009-27683 and Japanese Patent Application Laid-open No. 2008-36850, the tilt upon installation of the LPH to the image forming apparatus is considered, but a positional deviation among semiconductor chips inside the LPH is not considered. Furthermore, when using the methods disclosed in Japanese Patent Application Laid-open No. 2009-27683 and Japanese Patent Application Laid-open No. 2008-36850, scales of circuits required for control are increased and packaging thereof becomes difficult.
These problems are not limited to LPHs, and may also become problems for any light source of an optical writing device including plural chips connected in a main scanning direction.